Ion implantation is a physical process that is employed in semiconductor device fabrication to selectively implant dopant into semiconductor and/or wafer material. Thus, the act of implanting does not rely on a chemical interaction between a dopant and semiconductor material. For ion implantation, dopant atoms/molecules are ionized, accelerated, formed into a beam, analyzed, and swept across a wafer, or the wafer is swept through the beam. The dopant ions physically bombard the wafer, enter the surface and come to rest below the surface, at a depth related to their energy.
Referring to FIG. 1 ion implanters or ion implantation systems typically include three sections or subsystems: (i) an ion source chamber 102 containing an ion source for outputting an ion beam, (ii) a beamline assembly 110 including a mass analysis magnet for mass resolving the ion beam, and (iii) a process chamber 112 which contains a target location that receives the ion beam from the beam line assembly, such as a semiconductor wafer 114 or other substrate to be implanted by the ion beam. The continuing trend toward smaller semiconductor devices requires a beamline construction which serves to deliver contamination free and higher beam currents at all energies. The high beam current provides the necessary dosage levels, while the low energy permits shallow implants. Source/drain junctions in semiconductor devices, for example, require such a high current, low energy application.
Ion sources in ion implanters typically generate an ion beam by ionizing within the source chamber 102 a source gas, a component of which is a desired dopant element, and extracting the ionized source gas in the form of an ion beam. The ion source may take the form of an indirectedly heated cathode (IHC), typically utilized in low energy/high current, medium current and high energy ion implantation equipment.
When the ion source is operated using a molecular gas form of the desired material to be ionized and implanted into the substrate undesirable by-products of this gas ionization are produced. Properties of these gas species generated during the disassociation/ionization of the source gas are corrosive and/or highly reactive. These undesirable by-products result in damage to the internal and external mechanical and electrical components that are critical to the ion source performance. Some of these species, desirable or undesirable, may have very low vapor pressures, and as a result condense on the interior surfaces of the source as well as reacting with the construction materials of the cathode electrodes, repeller electrode and interior wall surfaces of the chamber. These solid deposits or chemical by products may interfere with ion source operation over time, for example by changing the electrical characteristics of the internal and external arc chamber electrical components, or partially blocking the ion source electrode aperture, thereby reducing the available ion current and detrimentally affecting the efficiency of the ion source and of the chamber 102.
Additionally, where fluorine-containing source gases are utilized, excess free fluorine radicals in the ion source chamber 102 can result in etching of the chamber housing material and internal components. The reactant(s) are highly volatile in nature at the ion source operating temperature. They either decompose or are pumped away. Fragile columnar structures build up due chemical etching and/or the halogen cycle and break off causing discharges by either bridging the cathode or repeller to ground or being ejected into the extraction to extraction suppression high voltage gap causing a discharge. This material can then be transported down the beamline to the wafer. It has been shown that material or debris that is generated inside the ion source chamber or picked up by the beam sweeping/modulating when it is interrupted by a discharge and may be transported to the substrate. These particulates have a direct effect on semiconductor device yield.
When running oxygen containing source gases, free oxygen radicals form oxides which will reduce cathode electron emission (poison) and lower required ion beam current.
In order to combat such effects, it has been known to run a co-gas with the source gases, thereby removing/minimizing undesirable species generated from the disassociation and ionization of source gases. Ion beam current, as well as lifetime of the ion source are thereby increased. Ion beam stability, particles and metal contamination are reduced. Thus, the ability to automatically control the selection and optimization of the flow rate of these co-gases would be desirable.